absence profiles in bacterial populations [57, 59] permits a more accurate evaluation of the genetic stability of a great number of bacterial pathogens.

To analyze genome plasticity in pathogenic and commensal E. coli isolates, Hacker and coworkers made use ofa whole-genome approach. Using DNA micro-arrays, the presence of all translatable ORFs of nonpathogenic E. coli K-12 was investigated in 26 extraintestinal (ExPEC) and intestinal pathogenic E. coli (IPEC) isolates, three pathogenicity island deletion mutants, and commensal and labora tory strains. In addition, they developed an "E. coli pathoarray," which consists of hundreds of probes specific for virulence-associated genes of ExPEC, IPEC, and Shigella, in order to evaluate the distribution of these genes among the pathogenic and commensal strains used [63].

In recent years, there have been enormous advances in DNA microarray technology and a remarkable amount of literature published supporting its central role in gene discovery and vaccine and drug development (Table 24.1). However, because the results of pathogen gene expression are influenced by the model system used, such results must be interpreted cautiously. Because of these considerations, traditional biological, pathology, and toxicity studies remain important.